落了一节课，直接lecture3

Lecture3 Basic Pipeline

1、Single-cycle implementation: seldom used

     取指  译码  执行  读mem/cache  写回WB

2、How to make it fast?

     To decrease the clock cycle time? difficult!

     What's the problem？资源浪费 串行 Start work ASAP

3、Multi-cycled implemention

4、Pipelining

    why pipelining？ decrease CPUtime  improving the throughput 

5、Pipeline hazard: the major hurdle

    A hazard is a condition that prevents an instruction in the pipe from excuting its next scheduled pipe stage

   Taxonomy of hazard:

   Structrural hazards: These are comflicts over hardzards resources.

   Data hazards: Instruction depends on result of prior computation which is not ready yet.

   Control hazards: PC are not available in time to fetch an instruction on the next clock.

6、 Hazards can always be resolved by stall

 7、浮点数运算：解决结构竞争（Structrural hazards）

      方法1：fully pipelined adder：overhead（体积、执行时间）过大

      方法2：multiple unpipelined adder：overhead（成本）过大

      方法3：插入stall

8、coping with data hazards

    方法1：增加数据通路（Forwarding Path）

    方法2：Forwarding not always work，so implement load Interlock ：load指令比较常见，这个方法开销较大

    方法3：编译器端,进行代码调度，进行代码块顺序改变